Method and apparatus for closed loop transmission

ABSTRACT

In a wireless communication system, a method and apparatus for closed loop transmission is disclosed. In accordance with the preferred embodiment of the present invention, a time frequency portion of an uplink frame is dynamically reserved as a sounding zone for uplink channel sounding. A first message is transmitted to a first subscriber station in a downlink frame assigning a time-frequency resource within the sounding zone, and a sounding waveform. Furthermore, a signal is received from the subscriber station within the assigned time-frequency resource, a partial channel response is determined from the received sounding signal, and the subsequent transmission to the subscriber station is tailored based on the at least partial channel response.

FIELD OF THE INVENTION

The present invention relates generally to communication systems, and inparticular, to a method and apparatus for closed loop transmission.

BACKGROUND OF THE INVENTION

In mobile broadband cellular communication systems, there are severalphysical layer techniques that require the transmitter to be providedwith knowledge of the channel response between the transmitter andreceiver. Transmission techniques that make use of the channel responsebetween the transmitter and receiver are called closed-loop transmissiontechniques. One example of closed-loop transmission is the use of aclosed-loop transmit antenna array at the transmitter. A closed looptransmit antenna array is an array of multiple transmit antennas wherethe signals fed to each antenna are weighted in such a way as to controlthe characteristics of the transmitted signal energy according to somepre-defined optimization strategy. Generally, the closed-loop transmitantenna array weights the transmitted antenna signals based on knowledgeof the space-frequency channel response between each transmit antennaand each receive antenna and attempts to optimize the characteristics ofthe received signal processed by the receiving device. For singleantenna transmitters, the transmitter can use the knowledge of thechannel to pre-equalize the channel so as to reduce or even eliminatethe need for complex receive equalization at the receiver. Havingknowledge of the channel response at the transmitter can also be used toselect the best modulation and coding rate to use when transmitting datato the receiver.

In general, there are two methods for providing a transmitter withknowledge of the channel between each transmit antenna and each receiveantenna. This discussion is focused at the downlink of a cellular systemwhere the base station (BS) is the transmitter and a subscriber station(SS) is the receiver.

The first method is based on feedback messages from the SS, where the SSmeasures the channel response between the BS antennas and the SSantennas and transmits a feedback message back to the BS containingenough information that enables the BS to reconstruct the downlinkchannel response and perform closed loop transmission. For example, theSS could feedback a quantized version of the downlink channel estimates.

The second method is based on the reciprocity of the RF channel responsein a TDD system. In a static (i.e., zero velocity) TDD system, the RFpropagation channel is reciprocal, which means the downlink RF channelmatrix (where the matrix refers to the channel gains between eachtransmit and receive antenna) at a given time-frequency point is simplythe matrix transpose of the uplink RF channel matrix at the sametime-frequency point. Therefore in a TDD system, a downlink channelresponse can sometimes be derived from an uplink data transmission ifthe data transmission includes pilot signals. However, in modern digitalcommunication systems, traffic (such as web browsing) is oftenasymmetric, meaning that there may not be an uplink transmissionassociated with each downlink transmission. Or, in a broadband system atypical uplink data transmission may be of a smaller bandwidth than thetypical downlink data transmission. These issues can severely degradethe performance of closed loop transmission in systems based onreciprocity. Therefore, there is a need for a closed loop transmissionmethods and associated signaling methods that can overcome theselimitations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a communication system.

FIG. 2 is a block diagram of a closed-loop transmit antenna arraycommunicating a single data stream to a receiving device having one ormore receive antennas.

FIG. 3 is a block diagram of a closed-loop transmit antenna arraycommunicating multiple data streams to a receiving device having one ormore receive antennas.

FIG. 4 is a block diagram of a frequency domain-oriented broadbandtransmission system employing a closed-loop transmit antenna array.

FIG. 5 is a timing diagram of Time Division Duplexing (TDD) framewherein time is divided into a downlink (DL) portion or frame and anuplink (UL) portion or frame and the downlink and uplink frames occupythe same frequency bandwidth.

FIG. 6 is a collection of uplink frame diagrams showing differentexamples of how a time-frequency portion of the uplink frame may bereserved as a sounding zone.

FIG. 7 is a diagram showing an example of how the sounding zone may bedynamically changed from one uplink frame to another.

FIG. 8 is a TDD frame diagram showing an uncoupled sounding methodwherein a base station (BS) transmits a message to a subscriber station(SS) in the DL frame instructing the SS to transmit a sounding signal inthe uplink sounding zone.

FIG. 9 is a diagram showing an uncoupled sounding method followed by aclosed-loop transmission in the next downlink frame.

FIG. 10 is a flowchart showing the operation of the uncoupled soundingmethod in the downlink.

FIG. 11 is a flowchart showing the operation of the uncoupled soundingmethod in the uplink.

FIG. 12 is a timing diagram showing the coupled sounding method whereina subscriber station (SS) is simultaneously given a downlink dataallocation including a command to transmit a sounding signal in theuplink sounding zone.

FIG. 13 is a timing diagram showing the coupled sounding method followedby a closed-loop transmission

FIG. 14 is a flowchart showing the operation of the uncoupled soundingmethod in the downlink.

FIG. 15 is a flowchart showing the operation of the uncoupled soundingmethod in the uplink.

FIG. 16 is a time-frequency diagram illustrating two methods ofachieving sounding signal separability within a same time-frequencyresource.

FIG. 17 shows an apparatus for providing closed loop transmission.

FIG. 18 shows an apparatus for performing channel sounding.

SUMMARY OF THE INVENTION

One aspect of the invention comprises a signaling method to provide atransmitting device with knowledge of the channel response between eachtransmit antenna and each receive antenna to enable closed-loop singleand multi-stream transmit array processing via uplink channel soundingwith assumed antenna array calibration. Several other applications arealso possible, for example: pre-equalization for single-transmit antennasystems; and determining the optimal modulation and coding scheme toemploy when transmitting to the receiving device. For simplicity, theinvention is presented from the point of view of providing a basestation (BS) with the channel knowledge needed for setting the transmitweights in a closed-loop antenna array system when transmitting to asubscriber station (SS). It should be clear that the invention appliesto scenarios where the roles of a BS and SS are reversed from the rolesdescribed herein. For example, the invention can be applied to thescenario where the SS is to be provided with channel knowledge to enableclosed-loop transmission from an SS to a BS. Therefore, although thedescription will focus mainly on the case of the BS performing closedloop transmission to a SS, the term “source communication unit” willrefer to a communication unit (e.g., a BS, SS or other transceiver) thatcan perform closed loop transmission to a “target communication unit”.

Also, some terms are used interchangeably in the specification: Theterms, channel response, frequency selective channel profile,space-frequency channel response, are all referring to the channelresponse information needed by the base station in order to utilizeclosed-loop transmission techniques. The terms waveform and signal arealso used interchangeably. A receiving device can be either a basestation (BS), subscriber station (SS) or any combination thereof. Also,a transmitting device can be either a BS, SS, or any combinationthereof. Additionally, if the system has repeaters, relays or othersimilar devices, the receiving device or the transmitting device can bea repeater, relay, or other similar device. The repeater or relay can beconsidered equivalent to an SS if the BS is performing closed-looptransmission to the repeater/relay. The repeater or relay can beconsidered equivalent to a BS if the relay is performing closed-looptransmission to the SS. The term fast Fourier transform (FFT) andinverse fast Fourier transform (IFFT) refer to discrete Fouriertransform (or similar transform) and inverse discrete Fourier transform(or similar transform) respectively.

One aspect of the invention is the dynamic reservation of a SoundingZone on the uplink. The Sounding Zone is a special region of the uplinkfor which SSs can transmit special sounding waveforms that enable thebase station to measure the uplink channel response, also known as afrequency selective channel profile. With hardware calibrationtechniques known in the art, the BS can determine the downlink channelresponse (or frequency selective channel profile) from the measureduplink frequency selective channel profile. Note that in some situationsan SS may transmit on only a subset of the uplink channel bandwidth (forexample, to overcome noise by increasing the power spectral density ofthe transmit signal with the same total transmit power), so the BS mayonly be able to determine and use a partial channel response for thatSS.

One aspect of the invention is optional signaling for determining thepresence and characteristics of the Sounding Zone. The BS can signal thepresence of the sounding zone by transmitting an information element(IE) in the uplink control channel (e.g., the UL-MAP in the IEEE 802.16standard). This IE can indicate the start time (generally the number ofbauds into the UL) and duration (generally measured in number of OFDMbauds). If the sounding baud is not intended to occupy the entire uplinkbandwidth, then the IE will contain information specifying the frequencyoccupancy of the sounding zone. Note that signaling to specify theSounding Zone is not required—rather, it is helpful for reducing theoverhead in the actual sounding assignments made to specific SSs.

One aspect of the invention is a signaling method for specifying thecharacteristics (i.e., the time-frequency sounding resource and theexact sounding signal) of the uplink sounding transmissions to be usedby the SS.

One aspect of the invention is a method for assigning multiple soundingwaveforms that are transmitted by different SSs (or by differentantennas on the same SS) in the same time-frequency resource, yet areseparable by the BS due to the properties of the waveforms. This enablesthe simultaneous sounding of multiple transmit antennas.

One aspect of the invention is a signaling method for assigning a SS totransmit a sounding signal in a specific time-frequency resource in theuplink frame.

One aspect of the invention is a signaling method that allocatesdownlink a data transmission to the SS and simultaneously assigns the SSto transmit a sounding signal in a particular time-frequency resource inthe UL frame.

Additional aspects of the invention are described in later sections ofthe specification. The present invention has a number of benefits: Thesignaling method provides a high level of flexibility and signalingefficiency. The method enables the dynamic allocation of an UL soundingresource to multiple SSs and handles the case where the SSs have singleor multiple transmit antennas. The method also provides multipletechniques for making the sounding waveforms separable by the BS, whichenables multiple SS antennas (whether the antennas are all on one deviceor are located on multiple SSs) to sound the UL on the sametime-frequency sounding resource. The signaling method is alsocomplementary with other methods of determining the UL channel response.For example, the sounding operations can be disabled (or turned off) ifthe BS can determine the UL channel response from the data transmittedby the SS on the UL. As part of the method for providing multipleseparable sounding waveforms, the signaling method provides the means todecimate the sounding transmissions in frequency so as to increase thetransmit power per subcarrier to better support SSs operating in low SNRenvironments. Also, the waveforms used for sounding transmission aredesigned to have low peak-to-average power ratio and to have the abilityto suppress other cell interference by virtue of their cross-correlationcharacteristics. Finally, the sounding waveforms provided by theproposed signaling method are designed to enable effective channelestimation at the BS so that the time variations in the channel responsecan be tracked.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, wherein like numerals designate likecomponents, FIG. 1 is a block diagram of communication system 100.Communication system 100 comprises a plurality of cells 105 (only oneshown) each having a base station (BS, or base station) 104 incommunication with a plurality of subscriber stations (SSs) 101-103. Ifclosed loop transmission is to be performed on the downlink to SS 101,the BS 104 can be referred to as a source communication unit, and the SS101 can be referred to as a target communication unit. If closed looptransmission is to be performed on the uplink from SS 101 to the BS 104,SS 101 can be referred to as a source communication unit, and the BS 104can be referred to as a target communication unit. In the preferredembodiment of the present invention, communication system 100 utilizesan Orthogonal Frequency Division Multiplexed (OFDM) or multicarrierbased architecture including Adaptive Modulation and Coding (AMC). Thearchitecture may also include the use of spreading techniques such asmulti-carrier CDMA (MC-CDMA), multi-carrier direct sequence CDMA(MC-DS-CDMA), Orthogonal Frequency and Code Division Multiplexing(OFCDM) with one or two dimensional spreading, or may be based onsimpler time and/or frequency division multiplexing/multiple accesstechniques, or a combination of these various techniques. However, inalternate embodiments communication system 100 may utilize othercellular communication system protocols such as, but not limited to,TDMA or direct sequence CDMA.

FIG. 2 is a block diagram of a closed-loop transmit antenna array aspart of a source unit communicating a single data stream to a receivingdevice as part of a target communication unit having one or more receiveantennas. Input stream 204 is multiplied by transmit weights 205 usingmultipliers 203 before being fed to the multiple transmit antennas 201.Multiplying input stream 204 by transmit weights 205, where the transmitweights are based on at least a partial channel response, is one exampleof tailoring a spatial characteristic of the transmission. Methods fordetermining the transmit weights from the channel response are known inthe art. The signals transmitted from the multiple transmit antennas 201propagate through a matrix channel 108 and are received by multiplereceive antennas 202. The signals received on the multiple receiveantennas 102 are multiplied by receive weights 206 using multipliers 203and summed by a summation device 209 to produce the output symbol stream207. In an embodiment where the transmitter has only one antenna, thespatial characteristic of the transmit signal cannot be tailored;however, other characteristics of the transmit signal may be tailoredbased on at least a partial channel response, such as the complex gainof each subcarrier (e.g., in a pre-equalization application), or themodulation and coding used on the subcarriers of the transmit signal.

FIG. 3 is a block diagram of a closed-loop transmit antenna array aspart of a source unit communicating multiple data streams to a receivingdevice as part of a target communication unit having one or more receiveantennas (e.g., a MIMO system). Multiple input streams 304 aremultiplied by transmit weights 305 using multipliers 303 before beingfed to the multiple transmit antennas 301. The signals transmitted fromthe multiple transmit antennas 301 propagate through a matrix channel308 and are received by multiple receive antennas 302. The signalsreceived on the multiple receive antennas 302 are multiplied by receiveweights 306 using multipliers 303 and summed by a summation devices 309to produce the multiple output symbol streams 307. Multiplying inputstreams 304 by transmit weights 305 where the transmit weights are basedon at least a partial channel response is another example of tailoring aspatial characteristic of the transmission. Other embodiments ofproducing the output symbol streams 307 are possible such as maximumlikelihood detection or successive cancellation that may or may not usethe receive weights 306 and the multipliers 303.

FIG. 4 is a block diagram of a frequency-domain oriented transmissionsystem such as Orthogonal Frequency Division Multiplexing (OFDM) orcyclic prefix single carrier (CP-Single Carrier) in which thetransmission techniques of FIG. 2 and FIG. 3 are performed in thefrequency domain prior to transmission. In a CP-Single Carrier system,one or more data streams 401 are first brought into the frequency domainwith one or more FFTs 402 and the frequency domain data streams areweighted with frequency domain weighting apparatus 403. In OFDM, the oneor more data streams 401 are sent directly to frequency domain weightingapparatus 403 without the use of FFT 402. The frequency domain weightingapparatus 403 implements the weighting function shown in the transmitportion of FIG. 2 and FIG. 3 on each subcarrier or frequency bin in thefrequency domain. Thus, the transmit signal can be tailored eitherspatially, or in frequency, or both with this type of a system. Theoutputs of the frequency domain weighting apparatus 403 are then broughtback into the time domain with IFFTs 404. Cyclic prefixes are added 405as is known in the art. Transmit filtering 406 is then performed beforesending the transmitted signals to the transmit antennas 407.

Specifying the Existence and Characteristics of the Sounding Zone

One aspect of the invention is the dynamic reservation of a SoundingZone within the Uplink (UL) of a Time Division Duplexing (TDD) Frame.FIG. 5 shows a timing diagram of one frame 501 of a TDD system, whichgenerally consists of two portions: a downlink (DL) frame or interval502 and an uplink (UL) frame or interval 503. Both the DL and the ULoccupy the same frequency band and are alternated in time. In acommunication system having a Base Station (BS), the DL is used fortransmissions by the Base Station, and the UL is used for transmissionsby the Subscriber Stations (SS). A TDD frame generally consists of oneDL frame followed by one UL frame, although variations are possible. Therelative length of the DL frame and the UL frame can be adjustedaccording to the expected relative levels of DL traffic and UL traffic.

The Sounding Zone is a special time-frequency portion of the uplink TDDframe that is dynamically reserved for the transmission of specialsounding waveforms by the SSs to enable the BS to measure at least apartial uplink frequency selective channel profile or channel response.To signal both the presence and characteristics of the sounding zone, aspecial information element (IE) may be transmitted by the BS in thecontrol channel (preferably the UL control channel, which for example inIEEE 802.16 is called the UL-MAP). For the purpose of explanation, thisIE will be referred to as the Sounding_Zone_Presence_IE( ), whichcontains the information that specifies exactly where the sounding zoneis located in time and frequency. Note that signaling to specify theSounding Zone is not required; however, it can reduce the overhead inthe actual sounding assignments made to specific SSs by broadcastingsome of the information that is useful for all SSs.

The allocated Sounding Zone may be in any time-frequency portion of theuplink frame. Placing the Sounding Zone early in the UL frame increasesthe available time for the base to process the received soundingwaveform. Placing the Sounding Zone late in the UL frame enables bettertracking of the channel time variations by providing more up-to-datechannel information (relative to the following TDD frames)

FIG. 6 shows several examples for where the Sounding Zone 602 can belocated within the UL frame 503. The Sounding Zone may be dynamicallyreserved in any time-frequency portion of the uplink frame 503. Placingthe Sounding Zone early in the UL frame increases the available time forthe base to process the received sounding waveform. Placing the SoundingZone late in the UL frame enables better tracking of the channel timevariations by providing more up-to-date channel information. Note thatthe sounding zone can be omitted from the UL portion of the frame simplyby not transmitting the Sounding_Zone_Presence_IE( ). Alternatively, theinformation in the Sounding_Zone_Presence_IE( ) can be transmitted withan indication that some number of the following frames will have aSounding Zone having the same characteristics as those being specifiedin the IE. This would eliminate the need to transmit aSounding_Zone_Presence_IE( ) in the control channel of every frame.

The Sounding Zone 602 can be dynamically reserved on a frame by framebasis as shown in FIG. 7. FIG. 7 shows multiple consecutive TDD frames501, each consisting of a downlink frame 502 and an uplink frame 503.The sounding zone 602 is shown to be scheduled in some but not all ofthe uplink frames in FIG. 7. Furthermore, some uplink frames are shownin FIG. 7 to have different time-frequency resources allocated to thesounding zone. The current sounding zone allocation can be signaled tothe SSs by updating and transmitting the Sounding_Zone_Presence_IE( ) ona frame-by-frame basis. By enabling dynamic reservation of the soundingzone, the BS is able to effectively adapt to different scenarios. Someexamples of this are as follows. In one example, the size of thesounding zone is selected based on an expected number of SSs that willneed to do sounding in the frame where the sounding zone is beingreserved. One aspect influencing this is that the number of active SSsover any time interval can vary, so the size of the sounding zone can bedynamically adjusted accordingly. Also, there may be a mixture of closedloop and open loop transmissions being made by the BS, so the size ofthe sounding zone may need to change accordingly. To accommodate thisand other scenarios while minimizing the overhead of the sounding zone,the size of the sounding zone may be changed from the current frame tothe next frame.

In a preferred embodiment for an IEEE 802.16-like system, the SoundingZone is constructed as follows. The signaling parameters of the OFDMAmode of the IEEE 802.16d air interface standard are used to provide adetailed example, but the invention is not limited to this particularexample.

The frequency band in the OFDMA mode of IEEE 802.16d is divided into 192frequency bins, where each frequency bin contains 9 OFDM subcarriers.For the 802.16 example:

To construct the frequency portion of the Sounding Zone, divide thefrequency band into 32 sounding frequency blocks, where each blockcontains 192/32=6 frequency bins. This means each sounding frequencyblock contains 54 OFDM subcarriers.

The Sounding Zone is allocated some number of OFDMA symbol intervals(also called bauds or OFDM symbol periods) that can vary on a dynamicbasis according to the level of traffic that will employ closed looptransmission.

For simplicity, the Sounding Zone is reserved either at the beginning ofthe UL frame or at the end of the UL frame. Alternatively, the exactlocation within the UL frame can be specified explicitly, but this wouldrequire additional signaling bits in the Sounding_Zone_Presence_IE( ).

According to the above preferred construction guidelines, the SoundingZone consists of a two dimensional grid of 32 sounding frequency blocksby some number of sounding bauds that can be dynamically adjustedaccording to the level of DL traffic that will use closed-looptransmission.

In a preferred embodiment for an 802.16-like system, theSounding_Zone_Presence IE( ) message contains the following information:

-   -   Control code indicating that this IE is a Sounding_Zone_Presence        IE( )    -   Length of the Sounding Zone in number of symbol intervals (3        bits)    -   Position of the Sounding Zone (1 bit). If set to one, then the        sounding zone is placed at the end of the UL frame. If set to        zero, the sounding zone is placed at the beginning of the UL        frame.

Note that the sounding zone can be arbitrarily placed at any symbollocation within the UL frame by specifying the starting symbol intervalof the Sounding Zone rather than using the simple one bit Positionfield. (More than one bit would be required by this approach.). In analternate embodiment, the sounding zone may allow some datatransmissions to occur within the sounding zone in addition to thechannel sounding transmissions if the data transmissions and soundingtransmissions are structured so that they do not significantly mutuallyinterfere with each other. This embodiment may be used in systems thatuse a form of multicarrier CDMA, where sounding transmissions and datatransmissions that occupy the same time-frequency resource are assigneddifferent and preferably orthogonal spreading codes.

Once the characteristics of the Sounding Zone are determined, there aretwo basic signaling methods for allocating time-frequency resourceswithin the Sounding Zone for SSs to transmit sounding waveforms to theBS. The first method is for the BS to generate and transmit a message,which may be transmitted in the control channel of the DL (preferably inthe UL control channel, also known as the UL-MAP in the IEEE 802.16dsystem) that assigns the time-frequency resource and the soundingwaveform (or sounding signal) to be used by the SS when sounding on theUL. The sounding waveform is preferably specified by a combination of aseparability mode, a separability parameter, and a sounding sequenceindex that are preferably included in the message. Other information,such as a BS identification number that is already known to the SS byother means, may also be used together with the message received by theSS to determine the exact sounding waveform to be used by the SS.

The second method is to piggy-back sounding instructions on a downlinkdata allocation in the DL control channel. The first method is called“Uncoupled Sounding” and the second method is called “Coupled Sounding”.These two methods are described in the following sections.

In an alternate embodiment, an explicit message (such as theUL_Sounding_Zone_Presence_IE( )) to reserve a portion of the UL forchannel sounding purposes is not used, and the reservation of an ULtime-frequency resource for sounding purposes is performed by the basestation without informing the SSs. In this embodiment, the BS simplygenerates and transmits a message assigning a SS a time-frequencyresource and a sounding waveform (or sounding signal) to be used by theSS when sounding on the UL where the sounding waveform is preferablyspecified by a combination of a separability mode, a separabilityparameter, and a sounding sequence index that are preferably included inthe message.

Uncoupled Sounding Method

In the first method, the BS reserves a time-frequency portion of an ULframe as a sounding zone for UL channel sounding, generates andtransmits a message to a SS in a DL frame. The message assigns orspecifies a time-frequency resource within the sounding zone, and asounding waveform. The sounding waveform is preferably assigned by aseparability mode, a separability parameter, and a sounding sequenceindex in the message.

One example of using this method for an 802.16-like system is to definean information element (IE), or message, preferably in the controlchannel (and preferably in the UL control portion) that contains theinformation assigning which time-frequency resource should be used inthe UL Sounding Zone and which sounding waveform should be used withinthat time-frequency resource. This information element that tells the SSthe time-frequency resource and the signal with which to sound withinthe Sounding Zone is called the UL_Uncoupled_Sounding_IE( ). Afterreceiving the UL_Uncoupled-Sounding_IE( ) addressed to the SS from theBS in a DL frame, the SS determines a sounding signal to be used andtransmits the sounding signal within the assigned time-frequencyresource in the UL frame. The details of how to specify thecharacteristics of the sounding signals/waveforms are presented later.

FIG. 8 contains a time-frequency diagram that shows the uncoupledsounding method for an 802.16-like system. In this figure, the controlinformation 803 transmitted in the DL 502 contains, among other things,an UL_Uncoupled-Sounding_IE( ) that is essentially an uncoupled soundingassignment message 808 that is generated and transmitted to the firstSubscriber Station (SS#1). This uncoupled sounding assignment message808, addressed to SS#1, tells the SS to transmit an uplink soundingtransmission 804 in the UL portion 503 of the frame by assigning atime-frequency resource within the sounding zone and a soundingwaveform. As shown in FIG. 8, note that the control information 803 canalso contain a data allocation for SS#1, which is simply a message thatreserves a portion 805 of the DL 502 for data transmission to SS#1. InFIG. 8, the data allocation message and subsequent transmission of thedata to SS#1 is unrelated to, or “uncoupled” from, the command totransmit sounding in the UL portion of the frame. In other words, theuncoupled sounding assignment message 808 (or UL_Uncoupled_Sounding_IE()) is independent of the data allocation message 810 in the controlinformation 803. The next section describes an alternative embodiment inwhich the data assignment for the DL and the uplink sounding assignmentare coupled together and transmitted in one information element withinthe control channel.

FIG. 9 contains a timing diagram of how the uncoupled soundingassignment message 808 enables closed loop transmission strategies onthe DL 502. First, the base transmits an uncoupled sounding assignmentmessage 808, (or UL_Uncoupled_Sounding_IE( )) in the UL control channel(control information 803) in the DL portion 502 of the first frame. Thismessage assigns a time-frequency resource within the sounding zone and asounding waveform to the SS. The SS then transmits an uplink soundingsignal 804 according to the instructions provided in theUL_Uncoupled-Sounding_IE( ) in the DL portion 502 of the first frame(based on the assigned time-frequency resource and the assigned soundingwaveform). The base receives the sounding signal 804 to estimate ordetermine at least a portion of the UL channel response, and transformsthe estimated uplink channel response portion to a Downlink channelresponse. This downlink channel response is then used to tailor acharacteristic of a subsequent transmission to the SS based on the atleast partial channel response to provide closed-loop transmission 904on the portion of the frequency band that coincides with the portion ofthe frequency band occupied by the uplink sounding waveform 804. Inother words, the closed-loop data transmission 904 is performed based onthe sounding waveform 804 received from the SS in the UL 503 of theimmediately preceding frame. In an alternative embodiment, theinformation learned from the sounding waveform 804 may be used in aclosed loop data transmission that is performed at any subsequent frame,not necessarily only at the immediately following frame shown in FIG. 9.One example for this scenario is when the channel varies slowly. In thiscase, the closed-loop transmission may be performed based on theinformation learned from any number of previously transmitted soundingwaveforms, the closed loop transmission will not be limited to only theportion of the frequency band that coincides with the portion of thefrequency band occupied by the uplink sounding waveform 804 received inthe UL 503 of the immediately preceding frame shown in FIG. 9. Instead,FIG. 9 just shows the timing diagram for high-mobility cases when themost prompt channel knowledge is desired. Examples of tailoring acharacteristic to the SS to provide closed-loop transmission include butare not limited to computing and applying complex transmit weights foreach transmit antenna (spatial transmit weights, computing and applyingcomplex transmit weights for each subcarrier of each antenna(space-frequency transmit weights), selecting an appropriatemodulation/coding scheme (adaptive modulation and coding), and computingand applying complex transmit weights to each subcarrier to compensatefor frequency selective fading (pre-equalization). Techniques forcomputing these types of transmit weights from a given channel responseare available in the art.

FIG. 10 is a flowchart showing the operation of the uncoupled soundingmethod in the downlink. The process begins with step 1001 wherein atime-frequency portion of an uplink frame is dynamically reserved as asounding zone for UL channel sounding. Flow proceeds to step 1002wherein a message is generated and transmitted to an SS in a downlinkframe, which assigns a time frequency resource within the sounding zoneand a sounding waveform to the SS for the SS to use in order to soundthe uplink channel. For an OFDM system, a time-frequency resource is aset of subcarriers and a set of OFDM symbol periods or bauds. Note thatthe subcarriers in the set are not required to be contiguous (see 602 ofFIG. 6 for example), and the symbol periods are not required to becontiguous.

FIG. 11 is a flowchart showing the operation of the uncoupled soundingmethod in the uplink. The process begins in step 1101 wherein the BSreceives a sounding signal from the SS within the assigned timefrequency resource in the sounding zone. Flow proceeds to step 1102wherein the BS determines at least a partial channel response (e.g.,channel response information for part of the frequency band) from thereceived sounding signal (or waveform). Finally in step 1103, the basetailors a characteristic (e.g., power, modulation and coding level,transmit antenna weights, spatial characteristics, etc.) of a subsequenttransmission to the SS based on the at least partial channel response toprovide closed loop transmission to the SS.

Coupled Sounding Method

A second embodiment for telling a SS to transmit a sounding waveform inthe UL is to piggy-back the sounding command on a DL data allocationmessage in the control channel (preferably the DL control channel, orDL-MAP in the terminology of IEEE 802.16d). The strategy is for the baseto transmit on the control channel one IE or control message, called theDL_Coupled_Sounding_Allocation_IE( ), that contains two parts: The firstpart reserves a portion of the DL frame for data transmission to aparticular SS. The second part provides an abbreviated set of soundinginstructions that will tell the SS to transmit sounding in the ULportion of the frame. To reduce the amount of signaling required tospecify the sounding waveform characteristics, the frequencycharacteristics of the sounding waveform are not explicitly included inthe DL_Coupled_Sounding_Allocation_IE( ), but rather are derivedimplicitly by the SS from the frequency characteristics of the DL dataallocation contained in the first part of the IE. This method has theadvantage of reducing the need to signal the frequency characteristicsof the sounding waveform. This method has advantages when the BS knowsthat the DL in the next TDD frame will contain a DL data allocation thathas the same frequency characteristics as the DL data allocation in thecurrent frame. In this situation, the BS can save some signalinginformation by transmitting the DL_Coupled_Sounding_Allocation_IE( ) andinferring the frequency characteristics of the requested soundingwaveform according to the frequency characteristics of the DL dataallocation in the DL.

FIG. 12 contains a timing diagram showing how the coupled soundingmethod operates. In contrast to FIG. 8, a single downlink allocationcontrol message 1215 or information element (IE) within the controlinformation 803 (control channel) simultaneously assigns the DL dataallocation 1205 for SS#1 and contains the information needed by the SSto transmit a sounding waveform 1206 on the same part of the bandoccupied by the downlink data allocation 1205. The sounding 1206transmitted by the SS is used by the BS to enable a closed-loop DL datatransmission in the DL of the next frame, not the current frame.

As shown in FIG. 13, the sounding transmission in the UL portion 503 ofthe first TDD frame is then followed by a DL closed-loop datatransmission 904 in the DL of the next TDD frame. A single downlinkallocation control message 1215 or information element (IE) within thecontrol information 803 (control channel) simultaneously assigns the DLdata allocation 1205 for SS#1 and contains the information needed by theSS to transmit a sounding waveform 1206 on the same part of the bandoccupied by the downlink data allocation 1205. The sounding 1206transmitted by the SS used by the BS to enable a closed-loop DL datatransmission 904 in the DL of the next frame, not the current frame. Inan alternative embodiment, the information learned from the soundingwaveform 1206 may be used in a closed loop data transmission that isperformed at any subsequent frame, not necessarily only at theimmediately following frame shown in FIG. 13. One example for thisscenario is when the channel varies slowly. In this case, theclosed-loop transmission may be performed based on the informationlearned from any number of previously transmitted sounding waveforms,the closed loop transmission will not be limited to only the portion ofthe frequency band that coincides with the portion of the frequency bandoccupied by the uplink sounding waveform 1206 received in the LL 503 ofthe immediately preceding frame shown in FIG. 13. Instead, FIG. 13 justshows the timing diagram for high-mobility cases when the most promptchannel knowledge is desired.

FIG. 14 is a flowchart showing the operation of the coupled soundingmethod in the downlink. The process begins with step 1401 wherein atime-frequency portion of an uplink frame is dynamically reserved as asounding zone for UL channel sounding. Flow proceeds to step 1402wherein a message is generated and transmitted to an SS in a downlinkframe, and this message assigns a time-frequency resource in the DLframe for the SS to receive DL data and also assigns a time frequencyresource within the sounding zone and a sounding waveform to the SS forthe SS to use in order to sound the uplink channel. In step 1402 thefrequency resource within the sounding zone is implicitly assigned basedon the time frequency resource assigned to the SS for receiving DL data.

FIG. 15 is a flowchart showing the operation of the uncoupled soundingmethod in the uplink. The process begins in step 1501 wherein the BSreceives a sounding signal from the SS within the assignedtime-frequency resource in the sounding zone. Flow proceeds to step 1502wherein the BS determines at least a partial channel response (e.g.,channel response information for part of the frequency band) from thereceived sounding signal (or waveform). Finally in step 1503, the basetailors a characteristic (e.g., power, modulation and coding level,transmit antenna weights, spatial characteristics, etc.) of a subsequenttransmission to the SS based on the at least partial channel response toprovide closed loop transmission to the SS.

Specifying the Sounding Waveform

As described earlier, in the preferred embodiment, the sounding zoneconsists of a time-frequency grid of 32 sounding frequency blocks bysome number of sounding bauds that can be dynamically adjusted accordingto the level of DL traffic that will use closed-loop transmission. Thissection describes how SSs are allocated time-frequency resource withinthe UL Sounding Zone and how SSs are told which waveform to transmit.

In the preferred embodiment, an SS that will be transmitting soundinginformation on the UL will be given a sounding allocation or assignmentwhich specifies the time-frequency resource within the sounding zonethat the SS will use to transmit the sounding waveform. In the preferredembodiment, the time-frequency resource given to an SS consist of acontiguous set of sounding frequency blocks across frequency within oneOFDM baud interval in the sounding zone. Other embodiments such asallocating non-contiguous frequency blocks or more than one soundingbaud are also within the scope of the invention. The important point isthat the time-frequency resource is specified by the soundingassignment.

In addition to the time-frequency resource to be used for sounding, theSS must known what specific transmitted signal or waveform that is to beused within the assigned time-frequency resource. In the preferredembodiment, this is accomplished by specifying several parameters of thesounding waveform, including:

-   -   A sequence index. This specifies the particular sequence that        the SS will use within the time-frequency sounding resource. It        is preferable to base the sequence on the Generalized Chirp-Like        (GCL) sequences as are known in the art because of their low        cross correlation properties. However, sequences other than GCL        sequences are possible to use. It is advantageous to use        different sets of GCL sequences to multiple co-channel base        stations so that the low cross correlation properties enable the        suppression of other GCL sequences being transmitted from other        co-channel cells. In the preferred embodiment, the GCL sequence        used for sounding is specified as follows. First, the SS will        calculate the length of the frequency domain sounding sequence        Ls, which is equal to the number of occupied subcarriers, from        the contiguous set of sounding frequency blocks (sounding band)        that the BS instructs the SS to sound, and the separability type        (or mode) and separability parameters (defined later), all        defined in the message UL_Uncoupled_Sounding_IE( ) received from        the BS. Then the SS determines the smallest prime number N_(G)        that is larger than Ls. The resulting sounding sequence is then:

${{s_{u}(k)} = {\exp\left\{ {{- j}\; 2\;\pi\; u\frac{k\left( {k + 1} \right)}{2N_{G}}} \right\}}},{k = {{0\;\Lambda\mspace{14mu} L_{S}} - 1}}$where k is the sequence element index and u is referred to as “sequenceclass index” that is calculated from the assigned sequence index and theCell ID of the BS as follows:

-   -   v1=1+decimal value of lowest 3 bits of the Cell ID    -   v2=1+decimal value of the sequence index    -   u=((v1)(v2)−1)mod(N_(G)−1)+1        In an alternative embodiment, the sequence index points to a        certain class index u in a pre-stored table. This u is used to        define the sounding sequence as above. A different table can be        assigned to different co-channels cells, either in a        pre-determined fashion or through dynamic allocation, to enable        the suppression of other GCL sequences being transmitted from        other co-channel cells.    -   A separability type (or mode) flag. This flag indicates the        separability method being used, where the separability method        refers to the method by which multiple transmitted sounding        waveforms can occupy the same time-frequency resource while        being separable at the BS receiver. The first separability type        is for the sounding waveform to utilize or occupy only a subset        of the subcarriers contained in the time-frequency resource.        Then, different SSs can use disjoint groups of subcarriers        within the time-frequency resource simultaneously without        interfering with each other's sounding signals. In the preferred        embodiment, disjoint groups of subcarriers are provided by using        a “comb” structure across the subcarriers belonging to the        sounding allocation. In other words, the sounding waveform only        occupies every N^(th) subcarrier within the time-frequency        resource. This type of occupancy across the subcarriers is a        form of interleaved frequency division, and may also be called        decimated subcarriers. Other SS sounding waveforms can occupy        the same time-frequency resource within the sounding allocation        but by occupying a different “starting offset” of every N'th        subcarrier across the sounding allocation. Separability is        achieved with this method by the fact that multiple sounding        waveforms occupy different sets of every Nth subcarrier across        the sounding allocation. This type of sounding is also called        frequency interleaving or decimated subcarriers, separability by        frequency decimation, or separability by frequency division, or        separating by interleaving the subcarriers occupied by the        different sounding waveforms. This type of sounding is        illustrated in FIG. 16 with elements 1601, 1602, 1603, 1604, and        1620. The second separability type is for the sounding waveform        to occupy every subcarrier of the time-frequency assignment, and        multiple sounding waveforms can occupy the same time-frequency        assignment as long as they have cross correlation properties        that enable the BS to estimate the channel response from each SS        transmitting within the same time-frequency assignment. This        type of sounding sequence separability is illustrated in FIG. 16        with elements 1611, 1612, 1613, 1614, and 1621. Note that in the        preferred embodiment that the separability flag indicates one of        two separability modes. However, the invention allows the        separability flag to possibly indicate more than two        separability modes if necessary.    -   Separability parameter: For the two types of separability        described above, the separability parameter is used as follows:        For frequency-interleaving or decimated subcarriers        separability, the separability parameter specifies a subcarrier        set offset index, (i.e., which of the N possible sets of every        N'th subcarrier is to be occupied by the sounding waveform). For        sequence separability across the sounding allocation, the        separability parameter provides additional information that is        used together with the sequence index to determine the specific        sounding waveform. In the preferred embodiment, the sequence        index determines a basic sequence to be used and the        separability parameter indicates a cyclic time shift amount that        is to be applied to the sounding waveform in the time domain        (after the IFFT in an OFDM system) prior to transmission. In        this case, the separability parameter indicates how to modify        the sequence derived from the sequence index to obtain the final        sounding waveform.    -   Multiple antenna parameter: For a SS with multiple transmit        antennas, this parameter indicates whether the SS should sound        on one antenna or on all of its transmit antennas.        Alternatively, this parameter could be expanded to explicitly        identify which set of antennas are to be sounded by the SS. If        this parameter indicates that multiple antennas to be sounded,        then the separability parameter is to be applied for antenna 1,        and the remaining transmit antennas are implicitly assigned to        employ subsequent separability parameters in sequence, while all        antennas use the same time-frequency resource.

FIG. 16 illustrates the two methods of the preferred embodiment forachieving separability in the sounding waveforms to be used on a portionof the sounding zone indicated by 1605. In an OFDM system as assumed inFIG. 16, the frequency band includes time-frequency elements 1606consisting of a single OFDM subcarrier within a single OFDM symbolperiod. In the interleaved subcarrier separability type, the sametime-frequency resource 1620 is assigned to four SSs (or possibly fourtransmit antennas on a single SS), but some of the elements 1606 of thetime-frequency resource are assigned to SS transmitter #1 (1601), someare assigned to SS transmitter #2 (1602), some are assigned to SStransmitter #3 (1603), and some are assigned to SS transmitter #4(1604). Because different elements 1606 are assigned to different SStransmitters, separability can be achieved at the BS receiver becausethe sounding transmissions occupy different (disjoint) sets of OFDMsubcarriers and are therefore orthogonal.

For the sequence orthogonality type method illustrated in FIG. 16, fourSS transmitters (1611-1614) transmit on the same set of subcarriers. SStransmitter #1 uses sequence 1 (1611), SS transmitter #2 uses sequence 2(1612), SS transmitter #3 uses sequence 2 (1613), and SS transmitter #4uses sequence 4 (1614). Separability is achieved at the BS by virtue ofthe sequences 1611, 1612, 1613, and 1614 having appropriatecross-correlation properties.

Note that in FIG. 16 that the different SS transmitters can all be onthe same SS or can be on different SSs, or on a combination of singleand multiple antenna SSs.

As noted above, the invention enables multiple SSs to perform channelsounding on the same time-frequency resource. In the preferredembodiment, when multiple SSs, for example two SSs, should be sounded,the BS will format and transmit a first message to a first SS, formatand transmit a second message to a second SS in the same DL frame, wherethe first and second messages assigning the same time-frequency resourcein the UL sounding zone, but assigning different waveforms to the firstand second SS. When using the preferred sounding waveform specificationapproach described above, the first and second SSs are assigned the sameseparability mode, but a different separability parameter.

When multiple SS transmitters (on the same or different SSs), forexample two SSs, are assigned the same time-frequency resource forsounding, the BS will receive a composite or summation of a first andsecond sounding signal transmitted from the first and second SS withinthe assigned time-frequency resource. The BS then determine at least apartial channel response for each of the first and second SS from thereceived composite sounding signal. This is enabled by assigningseparable waveforms to the SSs and processing the received compositesignal to separate the channel responses (e.g., by taking advantage ofthe disjoint subcarriers or the cross correlation properties). Afterobtaining the channel responses, the BS can tailor a characteristic of asubsequent transmission to at least the first SS based on the at leastpartial channel response of the first SS to provide closed looptransmission to the SS.

Apparatuses for Transmitting a Channel Sounding Message and forTransmitting a Sounding Signal

FIG. 17 is a block diagram of an apparatus within a source communicationunit containing a unit for transmitting a channel sounding message 1700plus a unit 1710 for tailoring a characteristic of a transmission to atarget communication unit to provide closed loop transmission. The unitfor transmitting a channel sounding message 1700 consists of areservation unit 1701, a sounding message unit 1702, and a transmitterunit 1703. The reservation unit 1701 dynamically reserves atime-frequency portion of a frame as a sounding zone for channelsounding. The sounding message unit 1702, which is coupled with thereservation unit 1701, generates a channel sounding assignment messagespecifying a time-frequency resource within the sounding zone, and asounding waveform to be used. The transmitter unit 1703, which isoperably coupled with the sounding message unit 1702, transmits thechannel sounding assignment message. The unit for tailoring acharacteristic of a transmission to the target unit 1710 consists of areceiver unit 1704, a channel response unit 1705, and a signal tailoringunit 1706. The receiver unit 1704 receives a sounding signal transmittedfrom a target unit within an assigned time-frequency resource. Thechannel response unit 1705, which is coupled with the receiver unit1704, determines at least a partial channel response from the receivedsounding signal. The signal tailoring unit 1706, which coupled with thechannel response unit 1705 and the transmitter unit 1703, tailors acharacteristic of a transmission to the target unit based on the atleast partial channel response.

FIG. 18 is a block diagram of an apparatus within a target communicationunit for transmitting a sounding signal 1800 and consists of a receiverunit 1801, a sounding message decoding unit 1802, and a transmitter unit1803. The receiver unit 1801 receives a channel sounding assignmentmessage from a source communication unit. The sounding message decodingunit 1802, which is coupled with the receiver unit 1801, determines fromthe sounding assignment message: a time-frequency resource in a frame,and a sounding signal, to be used for transmitting a sounding signal.The transmitter unit 1803, which is coupled with the sounding messagedecoding unit 1802, transmits the sounding signal within the assignedtime-frequency resource in the frame.

Messaging Formats in Preferred Embodiment

The following is a detailed description of the preferred messagingformats for an IEEE 802.16-like system that uses DL and UL maps toassign resources in the DL frame and UL frame, respectively. Note thatmost of the acronyms used in this section are defined in the IEEE 802.16family of specifications. In the UL-MAP (UL control channel), a BS maytransmit UIUC=15 with the UL_Sounding_Zone_Presence_IE( ) to indicate toall the SS the allocation of an UL sounding zone within the frame. TheBS may command a SS to transmit a sounding signal at one or more OFDMAsymbols in the sounding zone according to either the extended UL-MAPmessage UL_Uncoupled-Sounding_IE( ) or the extended DL-MAP messageDL_MAP_Coupled_Sounding_IE( ).

The definition of a sounding zone in UL_Sounding_Zone_Presence_IE( )makes it more efficient for later specifying the sounding symbol(s) thata SS will use to send the sounding signal(s), because only the relativeposition of the sounding symbol in the sounding zone can be specified.Otherwise, to indicate the absolute sounding symbol offset, a 10-bitfield would need to be used in each UL_Sounding_Zone_Presence-IE( ) toeach SS to be sounded.

Syntax Size Notes UL_Sounding_Zone_Allocatoin_IE( ){  Extended UIUC 4bits 0x03  Sounding Zone Length 3 bits Duration of the sounding zone (upto 8 OFDMA symbols)  OFDMA symbol offset 10 bits  Starting symbol of thesounding zone }

The SS-specific sounding instruction is transmitted from the base to aSS in UL_Uncoupled_Sounding_IE( ) where a CID is included. Thedefinition of the sounding zone is in UL_Sounding_Zone_Presence_IE( )that is targeted to all SS. The SS-specific messageUL_Uncoupled_Sounding_IE( ) instructs the SS to transmit specificsounding signal(s) at one or more specific symbol(s) within the soundingzone and to occupy specific frequency band(s) for each of these soundingsymbol(s). In this case, the sounding frequency allocations areindependent (uncoupled) from the presence or absence of any DL dataassignments to the SS in the DL-MAP.

Syntax Size Notes UL_Uncoupled_Sounding_IE( ){  Extended UIUC 4 bits0x04  CID 16 bits   Num_used_symbols 3 bits Number of sounding symbolsthis SS uses, from 1 (bits “000”) to 2³ = 8 (bits “111”)  for(i=0;i<Num_used_symbols;i++){  Sounding symbol index 3 bits Symbol indexin the zone, from 1 (bits “000”) to 2³ = 8 (bits “111”)  Startingfrequency band 5 bits Out of 32 (or 48) (or 6 bits) bands  Number ofbands 5 bits Contiguous bands (or 6 bits) used  Sounding sequence index2 bits Sequence index within a pre- defined 4-member group (severalgroups are pre- defined to be used in different sectors)  Separabilityflag 1 bit  0: sound all subcarriers in the assigned bands; 1: sounddecimated subcarriers  Length of Separability Parameter 3 bits Definethe length (“L”) of the next field, which varies from 1 (bits “000”) to2³ = 8 (bits “111”)  if (Separability flag==0) {   SeparabilityParameter Variable Cyclically shift   (Cyclic time shift index) lengththe time domain symbol by multiples (from 0 to 2^(L) − 1) of a CP length }  Else {   Separability Parameter Variable Relative starting  (Decimation offset) length offset position among the 2^(L)possibilities for the first sounding subcarrier   }  Multi-antennasounding mode 1 bit  0: sound the first antenna; 1: sound all theantennas, using the above- defined sequence as the starting shift ordecimation offset for the first antenna, and stepping through theremaining shifts or decimation offsets for each additional antenna  }

The SS-specific sounding instruction can also be transmitted from the BSto a SS as part of the DL-MAP information. The followingDL-MAP_Coupled_Sounding_IE( ) shall not be used unless theUL_Sounding_Zone_Presence_IE( ) is present in the same frame.DL-MAP_Coupled_Sounding_IE( ) commands the SS to transmit at one or morespecific symbols within the sounding zone using specific soundingsignals. In this case, the sounding frequency band information isderived from the DL allocations (thus the name “coupled sounding”).

Syntax Size Notes DL-MAP_Coupled_Sounding_IE( ){  Extended DIUC 4 bits If (INC_CID==1) {  N_CID 8 bits  For (n=0;n<N_CID;n++){   CID 16 bits  }  }  OFDMA Symbol Offset 10 bits    Subchannel offset 5 bits  Boosting 3 bits   No. OFDMA Symbols 9 bits   No. Subchanels 5 bits  Num_sounding_symbols 3 bits Number of sounding symbols this SS uses for (I=0;I<Num_sounding_symbols;I++){  Sounding symbol index 3 bitsSymbol index in the zone  Sounding sequence index 2 bits Sequence indexwithin a pre-defined 4-member group (several groups are pre-defined tobe used in different sectors)  Separability flag 1 bit  0: sound allsubcarriers in the assigned bands; 1: sound decimated subcarriers Length of the Separability Parameter 3 bits Define the length of (L)the next field, which varies from 1 (bits “000”) to 2³ = 8 (bits “111”) If (Separability flag==0) {   Separability Parameter VariableCyclically shift the   (Cyclic time shift index) length time domainsymbol by multiples (from 0 to 2^(L) − 1) of a CP length  }  Else {  Separability Parameter Variable Relative starting   (Decimationoffset) length offset position among the 2^(L) possibilities for thefirst sounding subcarrier  }  Multi-antenna sounding mode 1 bit  0:sound the first antenna; 1: sound all the antennas, using theabove-defined sequence as the starting shift or decimation offset forthe first antenna, and stepping through the remaining shifts ordecimation offsets for each additional antenna  }

The sounding frequency band information may also be uncoupled from theDL allocations, which is not shown here. In addition, uncoupled andcoupled sounding messages can be appended to existing 802.16d downlinkmessages to achieve some saving in messaging overhead, as shown in thenext two tables.

Syntax Size Notes MIMO_DL_Basic- Uncoupled_Sounding_IE( ){  ExtendedDIUC 4 bits 0x05  Length 4 bits Length in Bytes  Num_Region 4 bits  for( i = 0; i< Num_Region;i++) {  OFDMA Symbol offset 10 bits   Subchanneloffset 5 bits  Boosting 3 bits  No. OFDMA Symbols 9 bits  No.subchannels 5 bits  Matrix_indicator 2 bits STC matrix (see 8.4.8.4.)Transmit_diversity = transmit diversity mode indicated in the latestTD_Zone_IE( ). if (Transmit_Diversity = 01) { 00 = Matrix A 01 = MatrixB 10-11 = Reserved } elseif (Transmit_Diversity = 10) { 00 = Matrix A 01= Matrix B 10 = Matrix C 11 = Reserved }  Closed-loop MIMO flag 2 bits00 = Open-loop 01 = Closed-loop TxAA (meaning the following Num_layer= 1) 10 = Closed-loop MIMO 11 = Closed-loop SDMA (SS reacts by usingdifferent receive and channel estimation algorithms accordingly) Num_layer 2 bits  for (j = 0; j< Num_layer; j++){   if (INC_CID == 1) {  CID 16 bits    }  Layer_index 2 bits  }  }  Num_used_symbols 3 bitsNumber of sounding symbols this SS uses, from 1 (bits “000”) to 2³ = 8(bits “111”)  for (i=0;i<Num_used_symbols;i++){  Sounding symbol index 3bits Symbol index in the zone, from 1 (bits “000”) to 2³ = 8 (bits“111”)  Starting frequency band 5 bits (or 6 bits) Out of 32 (or 48)bands  Number of bands 5 bits (or 6 bits) Contiguous bands used Sounding sequence index 2 bits Sequence index within a pre- defined4-member group (several groups are pre-defined to be used in differentsectors)  Separability flag 1 bit  0: sound all subcarriers in theassigned bands; 1: sound decimated subcarriers  Length of Separability 3bits Define the length of the next Parameter (“L”) field, which variesfrom 1 (bits “000”) to 2³ = 8 (bits “111”)  if (Separability flag==0) {  Separability Parameter Variable Cyclically shift the time domain  (Cyclic time shift index) length symbol by multiples (from 0 to 2^(L)− 1) of a CP length  }  Else {   Separability Parameter VariableRelative starting offset position   (Decimation offset) length among the2^(L) possibilities for the first sounding subcarrier   }  Multi-antennasounding mode 1 bit  0: sound the first antenna; 1: sound all theantennas, using the above-defined sequence as the starting shift ordecimation offset for the first antenna, and stepping through theremaining shifts or decimation offsets for each additional antenna  }

Syntax Size Notes MIMO_DL_Basic- [Existing field inUncoupled_Sounding_IE( ){ MIMO_DL_Basic_IE( ) from accepted contribution80r1 shown in red]  Extended DIUC 4 bits 0x05  Length 4 bits Length inBytes  Num_Region 4 bits  for ( i = 0; i< Num_Region;i++) {  OFDMASymbol offset 10 bits   Subchannel offset 5 bits  Boosting 3 bits  No.OFDMA Symbols 9 bits  No. subchannels 5 bits  Matrix_indicator 2 bitsSTC matrix (see 8.4.8.4.) Transmit_diversity = transmit diversity modeindicated in the latest TD_Zone_IE( ). if (Transmit_Diversity = 01) { 00= Matrix A 01 = Matrix B 10-11 = Reserved } elseif (Transmit_Diversity =10) { 00 = Matrix A 01 = Matrix B 10 = Matrix C 11 = Reserved } Closed-loop MIMO flag 2 bits 00 = Open-loop 01 = Closed-loop TxAA(meaning the following Num_layer = 1) 10 = Closed-loop MIMO 11 =Closed-loop SDMA (SS reacts by using different receive and channelestimation algorithms accordingly)  Num_layer 2 bits  for (j = 0; j<Num_layer; j++){   if (INC_CID == 1) {   CID 16 bits    }  Layer_index 2bits  }  }   Num_used_symbols 3 bits Number of sounding symbols this SSuses, from 1 (bits “000”) to 2³ = 8 (bits “111”)  for(i=0;i<Num_used_symbols;i++){  Sounding symbol index 3 bits Symbol indexin the zone, from 1 (bits “000”) to 2³ = 8 (bits “111”)  Soundingsequence index 2 bits Sequence index within a pre-defined 4-member group(several groups are pre-defined to be used in different sectors) Separability flag 1 bit  0: sound all subcarriers in the assignedbands; 1: sound decimated subcarriers  Length of Separability Parameter3 bits Define the length of the (“L”) next field, which varies from 1(bits “000”) to 2³ = 8 (bits “111”)  if (Separability flag==0) {  Separability Parameter Variable length Cyclically shift the time  (Cyclic time shift index) domain symbol by multiples (from 0 to 2^(L)− 1) of a CP length  }  Else {   Separability Parameter Variable lengthRelative starting offset   (Decimation offset) position among the 2^(L)possibilities for the first sounding subcarrier  }   Multi-antennasounding mode 1 bit  0: sound the first antenna; 1: sound all theantennas, using the above-defined sequence as the starting shift ordecimation offset for the first antenna, and stepping through theremaining shifts or decimation offsets for each additional antenna   }Additional Information on Sounding Waveforms

The sounding waveforms are chosen to enable the BS to estimate theuplink channel for the frequency band of interest. Due to the limitedtransmit power of the mobile devices, especially handheld devices, it isimportant to improve the uplink link budget, which can be achieved bydifferent means such as power boosting allowed by transmitting onsmaller bandwidth and on decimated subcarriers only. Another importantmeans is to improve the PAPR of the OFDM-style sounding waveforms. Agood candidate for using as sounding waveform is the Generalized ChirpLike (GCL) waveforms which are a non-binary unit-amplitude sequences.For different sequence length, there are a number of GCL sequences(referred to as “classes”). When applied on uniformly spaced OFDMsubcarriers, the time domain signals also have unit amplitudes if thetime domain discrete time signal is at exactly the Nyquist samplingrate. But due to the guard subcarriers used in all practical OFDMsystems, the time domain waveform is equivalent to performingoversampling after a “sinc” pulse shaping filter. The resulting PAPRwill not have exact unit-amplitude, but a large number of the GCLsequences for any particular length still enjoy low PAPR (typically 3-4dB).

The GCL sequence used for sounding is expressed as

$\begin{matrix}{{{{s_{u}(k)} = {\exp\left\{ {{- j}\; 2\pi\; u\frac{k\left( {k + 1} \right)}{2N_{G}}} \right\}}},{k = {{0\Lambda\mspace{14mu} N_{G}} - {1\mspace{14mu}{and}}}}}{{u\left( {{\,^{``}{class}}\mspace{14mu}{index}^{"}} \right)} = {{1\Lambda\mspace{14mu} N_{G}} - 1}}} & (1)\end{matrix}$where N_(G) is the length of a GCL sequence (chosen as a prime number,explained later) and u is referred as the class index that is a non-zerointeger chosen between 1 and N_(G). The GCL sequence has the followingimportant properties:

-   Property 1: The GCL sequence has constant amplitude, and its    N_(G)-point DFT has also constant amplitude.-   Property 2: The GCL sequences of any length have an “ideal” cyclic    autocorrelation (i.e., the correlation with the circularly shifted    version of itself is a delta function)-   Property 3: The absolute value of the cyclic cross-correlation    function between any two GCL sequences is constant and equal to    1/√{square root over (N_(G))}, when |u1−u2|, u1, and u2 are    relatively prime to N_(G).

The cross-correlation 1/√{square root over (N_(G))} at all shifts(Property 3) is actually the minimum achievable value for any twosequences that have the ideal autocorrelation property (i.e., themaximum value of the cross-correlation at all shifts is minimized whichis equal to 1/√{square root over (N_(G))}). This property is importantwhen a number of potential interfering sequences are used, either in asingle sector or in a multi-sector environment. The cross correlationproperty allows the interfering signal evenly spread in the time domainafter correlating with the desired one. Hence, the channel of thedesired user can be detected more reliably (e.g., with a “de-noising”estimator).

The number of excited subcarrier in a sounding waveform is often not aprime number. For example, if a band of 36 subcarriers are sounded, thelength of the frequency-domain sounding sequence is 36. In this case, wepropose to choose the smallest prime number that is larger than thedesired length (e.g., 37 in above case), then truncating it to thedesired length. An alternative is to choose the largest prime numberthat is smaller than the desired length (e.g., 31 in the above case),then cyclically extending it to the desired length. When such amodification is performed, the three properties will only holdapproximately, but it is found that they still hold very well,especially when the sequence is long.

For any desired sounding sequence length (say L_(s)), as mentionedearlier, due to the use of oversampling effect introduced by applyingthe sequence only a contiguous subset of subcarriers, the PAPR will beincrease a little bit. But for each length value, the sequence classindices that give the best PAPR can be pre-stored. They can be dividedin to a number of groups (N_(gr), say 6) with each group consists of anumber of sequence classes (N_(cl), say 4). So, different group will beassigned to neighboring sectors and each sector can use one of theclasses (sequences) in its assigned group. The group assignment isconveyed from the BS sector to all users its serving area. In oneembodiment, each sector of a BS has an identification (ID) number. Thereis a one-to-one correspondence between the groups and the ID number.Therefore the sequences can be stored by each SS, and the appropriatesequence group can be selected by the SS based on the BS ID.

Although the sounding waveforms are assigned by the BS to SS in anorthogonal fashion, in which only a single sequence for each group isrequired, pre-assigning N_(cl) sequences for each group gives the optionto use more sequences when there are not enough orthogonal waveforms. Atleast, the additional sequences used will have minimalcross-correlation. Note that the each pair the sequences, no matter fromthe same group or from different groups, will have the minimum crosscorrelation property. Note that for best PAPR, a GCL sequence shouldexcite uniformly spaced subcarriers (spacing can be one). If an excitedsubcarrier falls at the DC subcarrier, the corresponding element of theGCL sequences should be used, although it can be set to zero before IFFTis taken.

Other than allocating a disjoint subset of the sounding subcarriers todifferent users to guarantee orthogonality, the other way to separatethe user channel in by transmitting a cyclically shifted version of thesame sequence by other users. For example, the first user transmit thesequence in (1) and the m-th user transmits the following sequence:s _(um)(k)=s _(u)(k)e ^(−j2nk(m−1)L) ^(CP) ^(/N),  (2)where S_(u)(k) is given in (1) and N is the FFT size (which is 2048) andL_(CP) is the cyclic prefix length.

The uplink channel estimation is discussed briefly here to illustratehow the cross correlation property can be taken advantage of. Basically,a time-domain estimator with adaptive tap selection is recommended wherethe time-domain channel response is obtained and only taps with a powerthat exceeds the noise power by a certain threshold will be included.This channel estimator can adapt to the instantaneous channel delayprofile, which is especially useful when the SNR is low and the channelis sparse. At moderate to high SNR cases, the MMSE channel estimator isalso satisfactory with similar performance to that of the time-domainapproach.

The time-domain uplink channel estimator is described as follows. Letthe frequency domain data for receive antenna n at the base is Yn(k)where k is a data subcarrier. Note that Yn(k) will consist of the uplinksounding for several users or transmit antennas. First, a noisy channelestimate is obtained by multiplying by a conjugate of the GCL sequenceas follows:{tilde over (H)}(k)=Y _(n)(k)S* _(u)(k)  (3)

Next the noisy estimates are transformed to the time domain through anN-point IFFT as:

$\begin{matrix}{{{\overset{\sim}{h}}_{n}(\lambda)} = {{\frac{1}{N}{\sum\limits_{k = 0}^{K - 1}{{{\overset{\sim}{H}}_{n}(k)}{\mathbb{e}}^{j\; 2\;\pi\;\lambda\;{k/N}}\mspace{14mu} 0}}} \leq \lambda \leq {N - 1}}} & (4)\end{matrix}$where K is the number of sounding subcarriers. N can use chosen as thesmallest integer than is bigger than K and is the power of 2. In thecase of sounding waveform as specified in (2), the different users' (ortransmit antennas') channels are separable in the time domain. Thismeans that an estimate of user m's time-domain channel is simply samples(m−1)L_(CP) through mL_(CP)−1 of (3). To improve the channel estimationin low SNRs, a tap-selection strategy is proposed. Tap selection simplymeans that the time taps that are below some threshold, η, are set tozero. Thus tap selection improves the channel estimation for relativelysparse channels by attempting to match the channel estimator to theinstantaneous power delay profile for each user. A threshold of η=3 dBstronger than the estimated SNR is an example of reasonable choices.

Let the time-domain channel estimate for all users after tap selectionbe denoted as ĥ_(n)(λ). Next denote the time-domain channel estimatesfor user m after the thresholding as h_(m,n)(λ) (where, explicitly, userm's channel is simply h_(m,n)(λ)=ĥ_(n)((m−1)L+λ) (0≦λ≦L−1)). Then thefrequency-domain channel estimate for user m is the N-point FFT ofh_(m,n)(λ):

$\begin{matrix}{{H_{m,n}(k)} = {{\sum\limits_{\lambda = 0}^{L - 1}{{h_{m,n}(\lambda)}{\mathbb{e}}^{{- j}\; 2\pi\;\lambda\;{k/N}}\mspace{14mu} 0}} \leq k \leq {K - 1}}} & (5)\end{matrix}$

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the spirit andscope of the invention, and that such modifications, alterations, andcombinations are to be viewed as being within the ambit of the inventiveconcept. It is intended that such modifications, alterations, andcombinations come within the scope of the following claims.

1. A method for enabling a base station (BS) to determine aBS-to-subscriber station (SS) channel response under an assumption ofchannel reciprocity, the method comprising the steps of: the basestation determining an uplink sounding zone within a frame, wherein theuplink sounding zone comprises a time and frequency resource; and thebase station instructing the SS to transmit a specific sounding signalat one or more specific symbol intervals within the uplink soundingzone, and the base station specifying a specific sounding frequency tobe occupied within each sounding symbol interval.
 2. The method of claim1 wherein the step of determining the uplink sounding zone comprises thestep of determining one or more OFDM/OFDMA symbol intervals in the framethat is used by the SS to transmit the sounding signal and enables theBS to determine a channel response between the BS and the SS.
 3. Themethod of claim 2 wherein the step of determining the uplink soundingzone comprises the step of determining non-overlapping frequency bands,wherein each frequency band comprises a plurality of consecutiveOFDM/OFDMA subcarriers.
 4. The method of claim 1 wherein the step ofinstructing the SS to transmit the specific sounding signal additionallycomprises the step of instructing the SS to transmit the sounding signalon consecutive subcarriers or decimated subcarriers.
 5. The method ofclaim 1 wherein the step of instructing the SS to transmit the specificsounding signal additionally comprises the step of providing the SS witha total number of sounding symbols to transmit.
 6. The method of claim 1wherein the step of instructing the SS to transmit the specific soundingsignal additionally comprises the step of instructing the SS to transmitthe sounding signal on consecutive subcarriers or decimated subcarriersand providing the SS with a decimation value (D) instructing the SS tosound every Dth subcarrier.
 7. The method of claim 1 further comprisingthe step of instructing a second SS to transmit a specific soundingsignal at the same symbol interval within the sounding zone, andspecifying a specific sounding frequency to be occupied by the second SSwithin each sounding symbol interval.
 8. The method of claim 7, furthercomprising the steps of receiving a composite of a first and secondsounding signal transmitted from the first and second subscriber stationwithin the assigned time-frequency resource; determining at least apartial channel response for the second subscriber station from thereceived composite sounding signal; and tailoring a characteristic of asubsequent transmission to the second subscriber station based on the atleast the partial channel response of the second subscriber station. 9.The method of claim 1 further comprising the steps of: receiving thesounding signal transmitted from the SS within the assigned soundingzone; determining at least a partial channel response from the receivedsounding signal; and tailoring a characteristic of a subsequenttransmission to the SS based on the at least partial channel response.10. The method of claim 9 where the step of tailoring comprises the stepof tailoring at least one of: a frequency characteristic of thesubsequent transmission, and a spatial characteristic of the subsequenttransmission.
 11. The method of claim 1 further comprising the step ofassigning a time-frequency resource in the downlink frame for the SS toreceive downlink data, and wherein a frequency part of thetime-frequency resource within the sounding zone is assigned implicitlybased on the time-frequency resource in the downlink frame for the SS toreceive downlink data.
 12. The method of claim 1 wherein the basestation additionally instructs the SS on a separability type identifyinga technique for maintaining signal orthgonality between multiplesounding transmissions.
 13. The method of claim 12 wherein the basestation additionally instructs the SS on a separability parameteridentifying a cyclic shift of the specific sounding signal.